1. Field of the Invention:
The present invention relates to a resistance measuring mechanism in a resistance welding machine for calculating a substantial resistance value by detecting voltage, welding current, temperature at an arbitrary portion of the resistance welding machine.
2. Description of the Prior Art:
It is necessary to calculate a substantial resistance value for detecting deteriortion of a cable or wearing of tip and the like in a welding machine. The resultant resistance value has been conventionally detected by calculating voltage as well as welding current between both ends of the cable or tips, e.g. as explained in Japanese Patent Laid-Open Publication No. 61-189882.
However, there arises a problem to judge the deterioration of the cable and the like (welding apparatus as a whole including welding gun) in a practical extent on the basis of the calculated resistance value although it is possible to calculate the resistance value of the cable and the like with high accuracy based on the values of the voltage and the welding current. That is, an electric resistance of a conductor such as a copper wire in general used in the cable and the like is varied depending on the temperature of the conductor. The temperature of the conductor is varied depending on the cold water for cooling the cable and the like at seasons, times or places where cold water is used. Furthermore, the temperature of the conductor increases at the weld time and the rate of increase of the temperature depends on the current value, amount and temperature of the cold water, timing of measuring the temperature of the conductor, etc.
Hence, in the conventional method for calculating the resistance value of the cable and the like on the basis of only the voltage and current values, the resistance value is affected by variation of resistance due to deterioration of the cable and variation of temperature of the conductor, which entails to create problem.